1. Field of the Invention
The present invention relates to a servo motor control apparatus and, more particularly, to an apparatus effecting feedback control of a servo motor according to an external operation command.
2. Description of the Related Art
Servo motors are widely used in various kinds of numerical control (NC) machine tools, working robots, measuring apparatuses, or the like, and an apparatus controlling the servo motor effects a feedback control according to an external operation command. For example, when the servo motor control apparatus receives information of an aimed rotational position and an aimed rotational speed as the operation command, it computes a command value of drive current (hereinafter referred to as a current command value) to be fed to the servo motor based on the above aimed values, an actual rotational position and an actual rotational speed of the servo motor, and multiplies a deviation between the current command value and current actually flowing in the servo motor (hereinafter referred to as an actual current) by a predetermined coefficient (feedback gain) to carry out the feedback control. The servo motor produces rotational magnetic fields upon receipt of the drive current from the control apparatus and causes its rotor to be rotated in synchronization with the rotational magnetic fields.
As the rotational speed of the servo motor is increased, however, the actual current lags by a certain phase amount behind the current command value. Namely, with the increase in the rotational speed, the phase difference between a power supply voltage applied to the servo motor and the actual current is made larger and thus so-called reactive power components are increased. Note, the current command value is synchronous with the power supply voltage.
Although the current command value is controlled so that it takes an optimum phase based on the aimed values and the feedback values, the actual current (i) is represented, using the maximum value of the power supply voltage (V m), the angular frequency (.omega.), the resistance (r) and inductance (L) of the motor winding, by the following formula: ##EQU1## where .theta.=tan.sup.-1 (.omega.L/r).
Therefore, the actual current i lags by a phase difference .theta. compared with the current command value corresponding to the rotational position of the servo motor. Note, this phase difference fluctuates somewhat according to individual characteristics of each motor.
FIG. 1 illustrates the above relation, in which a solid line represents the current command value; a broken line represents the actual current (i); and a dashed line represents the phase difference (deviation) between the current command value and the actual current (i). Also, a hatched portion shows an reactive component which does not contribute to the rotation of the servo motor among the current fed to the servo motor. According to a conventional servo motor control apparatus, the feedback gain is selected so that the reactive component is minimized.
Referring to the above formula, however, since the phase difference (phase lag) .theta. is changed depending on the rotational speed (.omega.), it is very difficult to always select an optimum feedback gain with respect to all of the rotational speeds and, accordingly, it is only possible to select an optimum value for a rotational speed which is utilized most frequently.
As a result, a drawback occurs in that, especially when the rotational speed is made higher, the reactive components are increased resulting in a lowering in efficiency of the servo motor. Namely, the percentage at which the current fed to the servo motor contributes to a torque is decreased and thus calorific power is relatively increased.
Also, when the servo motor rotates, counter electro motive force (E.M.F.), i.e., induced voltage, is generated in an armature winding coil of each phase of the servo motor. This counter E.M.F. (E m) is generated in the direction in which it counterbalances the current fed to the servo motor, and represented, using the number of turns (N) of the coil and the magnetic flux (.PHI.) linking to the coil, by the following formula: EQU Em=N.multidot.d.PHI./dt.
where .PHI.=.PHI.m.multidot.sin.omega.t, in which .PHI.m indicates the maximum value of the magnetic flux. Namely, the counter E.M.F. (E m) is proportional to the rate of change in the magnetic flux, i.e., the rotational speed of the servo motor.
As a result, a problem occurs in that, especially when the rotational speed is made higher, the voltage actually applied to the servo motor is lowered by the influence of the counter E.M.F.(E m) and, accordingly, the actual current becomes smaller than the current command value. Therefore, the torque of the servo motor is made small compared with the aimed value indicated by the operation command.
To cope with this, a conventional servo motor control apparatus detects the actual armature current and effects the current feedback control such that the aimed current flows in the servo motor.
However, since the current feedback control must be carried out at the highest speed in the servo system as described later, a burden on the servo motor control apparatus is made heavy. Also, sensing elements for detecting the armature current needs to be provided and, accordingly, the constitution of the control apparatus relatively becomes complicated.
On the other hand, an apparatus effecting a pulse width modulation (PWM) control is known which compares a torque command value with a feedback value, substitutes the difference between their values for a predetermined function to determine a PWM command value, and feeds three-phase alternating current having a magnitude corresponding to the determined PWM command value to the servo motor by means of the PWM control. For example, an example of the apparatus employing the PWM control is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 63-148891, which is intended to improve a response of the control system including a servo motor by setting the function for determining the PWM command value in consideration of resistance and inductance of the armature coil of the servo motor.
However, even if the above constitution is intended to improve the response of the system by regulating the function for the PWM command value and the current feedback gain, a disadvantage arises in that the constitution cannot be adapted to changes in individual characteristics of various motors to be controlled and thus the improvement of the response is limited. This is because the characteristics of the servo motor are changed by the change in running states to cause the change in energy losses and, even if the current corresponding to the computed PWM command value is fed to the motor, the drive power required for the motor is not generated. Thus, where the drive power required for the motor is not generated, the output torque of the servo motor fluctuates microscopically even if it is constant macroscopically. As a result, for example, where the servo motor is used as a driving source of a tool of a machinery processing apparatus, a drawback occurs in that, when an extremely precise processing is carried out, nonuniformity is formed on the processing surface.